Using the internet is only worthwhile if you can show off the perverse and wonderful things you've found on it. So, now that we're always online and always in touch, we're constantly sharing links. But what if you've seen it already? Lie.
You've probably encountered this already: most of your friends are bored all day, and so they'll hop from blog to blog collecting GIFs and funny reads. Then, because everyone thinks they're the only ones with access to the World Wide Web, your friends will pass the links around. Sometimes with a OMG have you seen this? GChat IM. Sometimes with a wall post. Sometimes just a bare URL in an email.
But you. You've seen it already. You've already found the [Cute dog playing with cactus/link to movie trailer/Shit Birds Say YouTube video/funny GIF of Drake] on your own. It's not new.
So what do you say to your friend?
Here he is, arms outstretched, face contorted into a huge proud grin, link in hand. Click it, won't you? Oh, click it and love it!
Let them down easy.
For every link you've seen, you've passed off plenty of old ones yourself. As much as you want to assert your web hegemony and denounce, with all the contempt of a hundred thousand Tumblrs, Old.
I've seen it.
Yeah I saw that.
With your nose in the air, an imperial hand waving the link away, you dismiss your friend. Seen it. Been there. Boring. Find something cool, and come back to me, okay?
Don't. Don't ever. As much as you might want to indulge the internet-inherent urge to look better than everyone, don't gratify that impulse -- it can be stifled with just the slightest civility. Fib your way through.
Don't do that. Do this instead.
"haha" (make sure it's not a plane crash or something sad)
"lol" (make sure it's not an invitation to a funeral)
"I love it" (make sure it's not another plane crash)
Or, if you don't want to risk laughing at a plane crash or can't stomach a white lie to a friend, it is possible to acknowledge an old link.
"Oh, yeah, I saw that!"
"I know, I can't believe it either!"
"Ah I love that! Have you seen [insert related/unrelated cool thing]."
The gentle nod approach will give your partner the satisfaction online existence acknowledgement without the stinging pain of being a few minutes behind the zeitgeist. If we can't IM each other the resplendent ocean bounty of the internet, we'll turn against one another. Spite and resentment will rule the day, not rage faces and cat breading. An internet with less sincerity might sully our souls, but an internet across which shooting links isn't safe from reproach is the worst hell of all.
User Manual is Gizmodo's guide to etiquette.